1. Field of the Invention:
This invention relates to small sized zoom lenses and more particularly to small sized zoom lenses in a range from a semi-wide angle to a semi-telephoto suited for compact cameras such as lens shutter cameras.
2. Description of the Prior Art:
Following the recent trend in the art to reduce camera bulk and size, there has been an increasing demand for shortening the objective length in the longitudinal direction. And for camera having a lens shutter, which is not able to interchange its objective lens, a zoom lens is desired instead of the monofocal lens. In this case, the size of the zoom lens should not be unduly larger than that of the monofocal lens.
Many zoom lenses of a range including the standard field angle (letting 2.omega. denote the total angular field, the semi or standard field angle is 2.omega.=47.degree. for the 35 mm still camera with 50 mm lens), the semi-wide angle and the semi-telephoto angle (hereinafter called "standard" zoom lenses) have, for example, been proposed in Japanese Patent Publication No. SHO 49-29146. The zoom lens of the kind described in this patent includes a front lens group of negative power and a rear lens group of positive power, the air space between the front and rear lens groups vary to bring about zooming. The use of such negative power followed by positive refractive power distribution, however, necessarily results in a long back-focal distance which, though suitable for single lens reflex cameras, is too large for a small size lens shutter camera.
To remove such a drawback, Japanese Laid-Open Patent Application No. SHO 57-201213 has proposed a zoom lens in which the front lens group is positive and the rear lens group is negative. With this power distribution, it is relatively easy to achieve a zoom lens system with a shortened back-focal distance as well as a shortened physical length, as will be described later. In the practical embodiment of the prior art, however, two lens elements of positive and negative powers are used to form the rear lens group of the zoom lens. For use in simple cameras, the number of lens elements is rather large. This makes it difficult to construct a compact zoom lens.
An object of the present invention is to provide a of reduced length zoom lens from the front vertex to the image plane.
A second object is to provide a zoom lens composed of a relatively small number of lens elements.
A third object is to provide a lens having a zooming range from the standard angular field to the wide angular field.